Polymeric materials are used widely to package and transport a wide variety of consumer goods, including perishable items which must be transported and sold at refrigerator or freezer temperatures. Commercially popular polymeric materials include polyethylene, styrene/butadiene copolymer and polystyrene.
At refrigerator or freezer temperatures, pure polystyrene is unsatisfactory for transporting perishable goods because it is brittle and fails during transportation. A known method of increasing the low temperature impact strength of polystyrene is the addition of rubber to the polystyrene composition. Typically, general purpose polystyrene (GPPS) is blended with high impact polystyrene (HIPS), which usually contains around 7% rubber particles by weight, to form a GPPS/HIPS admixture.
The addition of rubber to a polystyrene composition results in increased impact strength but increases the opacity of the polystyrene. Clear containers for packaging goods are desirable because it allows the customer to see the goods before purchasing. Typically, the percentage of HIPS in polystyrene cannot exceed 1% (0.07% rubber content) because polystyrene compositions with more than 1% HIPS show improved impact resistance but are too hazy for commercial applications. A polystyrene composition comprising 99% GPPS and 1% HIPS is relatively clear but does not have a sufficiently high low temperature impact strength and therefore is not suitable for use at low temperatures. There is a need for a polystyrene composition comprising more than 1% HIPS (0.07% rubber content) which provides higher impact strength at room and low temperatures and is clear.
Polystyrene compositions are typical of polymeric materials in that clarity and strength can be improved by orienting the polystyrene molecules. Orientation of polystyrene compositions is accomplished by stretching the GPPS/HIPS admixture. Polymeric compositions can be stretched biaxially. The two axial directions are typically perpendicular to each other. Biaxial stretching improves the clarity of polymeric compositions and increases the impact strength. Biaxial stretching of polystyrene compositions is known (Ayres et al, U.S. Pat. No. 3,995,763). Ayres, however, provides an opaque composition. Furthermore, present commercial biaxially oriented polystyrene compositions are unsatisfactory because of poor impact strength. Thus, there is a need for a clear biaxially stretched polystyrene composition with improved impact strength.